1. Field of the Invention
The present invention relates to controlling a flying height of a head of a hard disk drive.
2. Background Information
Hard disk drives contain a plurality of magnetic heads that are coupled to rotating disks. The heads write and read information by magnetizing and sensing the magnetic fields of the disk surfaces. There have been developed magnetic heads that have a write element for magnetizing the disks and a separate read element for sensing the magnetic fields of the disks. The read element is typically constructed from a magneto-resistive material. The magneto-resistive material has a resistance that varies with the magnetic fields of the disk. Heads with magneto-resistive read elements are commonly referred to as magneto-resistive (MR) heads.
Each head is attached to a suspension arm to create an subassembly commonly referred to as a head gimbal assembly (“HGA”). The HGA's are attached to an actuator arm which has a voice coil motor that can move the heads across the surfaces of the disks.
Each head has an air bearing surface that cooperates with an air flow generated by the rotating disk to create an air bearing. The air bearing prevents mechanical wear between the head and the disk.
The MR heads are typically constructed from a material that is sensitive to temperature. Most commercially available MR heads undergo a significant degradation of performance when the head temperature falls below a threshold value. This head degradation may increase the bit error rate of the disk drive to an unacceptable level.
Some drives contain heads that have integral heating elements. The heating elements generate heat so that the heads operate in a desirable temperature range. FIGS. 1 and 2 show a prior art heater control circuit 1 and corresponding timing diagram, respectively, for a heating element of a head. The control circuit 1 contains a pair of registers 2 and 3 within a preamp 4 of a disk drive. The register values are provided by a controller 5. The controller 5 controls the read and write operations of the disk drive. The contents of the registers 2 and 3 establish an amplitude of a voltage supplied to a heater 6 of a head.
The amplitude of the voltage controls how much heat is generated by the heater 6. The output of the registers 2 and 3 is converted from digital to analog form by a digital to analog converter (“DAC”) 7 and amplified by driver circuit 8. The contents of register 2 determines the value of the voltage during a read operation and the contents of register 3 determines the voltage amplitude during a write operation. The use of either register 2 or 3 is selected by the controller thru load circuit 9 and corresponding R/W signals.
As shown by the timing diagram of FIG. 2, during a write operation, a voltage of approximately 2 V is applied to the heater. In a write operation, a current is provided to a write element of the head. The current generates heat that increases the temperature of the head. The increase in temperature reduces the flying height of the head as shown in the diagram. During a read operation current is no longer provided to the write element. The read element receives a nominal biasing current but this does not generate any appreciable amount of heat. Consequently, the head temperature falls and the flying height increases. The voltage to the heater is increased to minimize the variation in flying height between the read and write operations. A subsequent write operation will cause a corresponding rise in temperature and a reduction in the flying height.
The variation in flying height during the write operation complicates the writing process and may even result in corrupt data. It would be desirable to minimize the fluctuations in flying height during and between the read and write modes for a head with a heating element.